Low-voltage circuit breakers are known as protective devices and are also used for protection of asynchronous motors as specific loads. They are used to protect the asynchronous motors against overloading and to protect the power distribution installation against short-circuiting. Particularly in the event of overloading, the asynchronous motors may be thermally overloaded. In order to prevent this, a tripping time at a reference current is set, inter alia, as a parameter on the overcurrent release of the circuit breaker, from which tripping time the delay before tripping of the low-voltage circuit breaker is obtained, depending on the overcurrent that is flowing.
In accordance with the appropriate standard, the tripping time can be selected from a time band. In addition to the tripping time, the parameters for the overload current and for the short-circuit current can also be set. In the case of asynchronous motors, their specific characteristics must be taken into account, and in practice this is done by way of the abovementioned setting parameters.
Although the motor manufacturers specify the technical data for the asynchronous motors (rating, starting current in comparison to the rated current, etc.) these do not, however, allow optimum setting of the overcurrent release, and therefore of the low-voltage circuit breaker. The setting of the overcurrent release is made even more difficult because of the fact that the setting parameters of the protective device depend on the load. Different characteristics for the individual asynchronous motors also make the setting process more difficult. In general, the parameters are therefore set by trial and error, that is to say the parameters are increased until the protective device no longer trips prematurely.